The inventive concept relates to nonvolatile memory devices and writing methods for nonvolatile memory devices. More particularly, the inventive concept relates to resistive nonvolatile memory devices and writing methods for resistive nonvolatile memory devices.
Volatile memory cells, like those used in dynamic random access memory (DRAM), as well as certain nonvolatile memory cells, like those used in flash memory, store data in accordance with a characteristic electrical charge. In contrast, other nonvolatile memory cells store data in accordance with a characteristic resistance that may be varied and detected by applying certain control signals (e.g., voltage(s) and/or current(s)). Nonvolatile memory devices that incorporate resistance-variable nonvolatile memory cells include, at least; phase-change random access memory (PRAM), resistive random access memory (RRAM), and magnetic random access memory (MRAM).
PRAM typically uses one or more phase-changeable material(s), such as chalcogenide alloy, to indicate a stored data state. RRAM typically uses one or more resistance-changeable elements, such as a magnetic tunnel junction (MTJ) thin film, to indicate a stored data state, and MRAM typically uses the magnetization properties of a ferromagnetic material to indicate a stored data state.
More particularly, the phase-change material of a PRAM may be placed into a crystalline state or an amorphous state by the carefully timed application of heat inducing control signals, wherein the crystalline state of the phase-change material—commonly interpreted as a set (or “0” value) data state—exhibits a relatively low resistance, while the amorphous state of the phase-change material—commonly interpreted as a reset (or “1” value) data state—exhibits a relatively high resistance.
The sequence of operative steps generating and applying control signals to a selected group of one or more nonvolatile memory cells in order to define one or more data states to be stored by the nonvolatile memory cells may be termed a “writing” method. The typical object of a writing method directed to a resistance-variable, nonvolatile memory cell is one in which the characteristic resistance manifest by the memory cell is placed within a defined resistance distribution (e.g., a range of resistances extending from a lower limit to an upper limit). Effective writing methods must balance the accuracy with which nonvolatile memory cells are written with the time required to execute the writing method.